The both complementary and alternative medicine practice of therapeutic fasting is recommended as a treatment of obesity and other metabolic diseases. Obesity is currently the most significant nutritional disorder in the United States and a reduction in the prevalence of obesity is targeted as a major objective in "Healthy People 2010". However, despite these goals, the prevalence of obesity is continuing to rise. New treatment options for obesity are desparately needed. The weight loss and metabolic benefits of traditional caloric restriction tend to be small and short lived. There are strong data to support a variety of benefits and durability of weight loss with therapeutic fasting. However, persistent fasting is not practical and has increased associated adverse events. Recently, animal data suggests that intermittent fasting (every other day) has similar metabolic benefits to prolonged fasting. There are also data to suggest that intermittent fasting will be safer than prolonged fasting and easier to tolerate than traditional caloric restriction. The studies described herein are designed to test the overall hypothesis that therapeutic intermittent fasting (IF) will be a more effective treatment for obesity than moderate daily caloric restriction (CR). The following specific aims will be addressed: 1) To determine the effect of IF compared to CR on weight loss and body composition 2) To explore the effect of IF compared to CR on insulin sensitivity, lipid levels, and blood pressure 3) To evaluate the safety and tolerability of IF compared to CR Following baseline measurements, participants will be randomized to a standard caloric restriction diet or to intermittent fasting. Interim measures will be collected at one week, and follow up measures collected at eight weeks. Exploratory data will be collected at 20 and 32 weeks to evaluate the durability of the intervention and monitor long-term safety.